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Title:- Signal Conditioning
- 1. 1 Mizan-Tepi University College ofEngineering & Technology Department of Mechanical Engineering COURSE NAME: INSTRUMENTATION & MEASURMENT CHAPTER FOUR – Signal conditioning By: Mr. Belachew Girma
- 2. • Signal conditioningis concerned with improving the quality of the reading or signal at the output of a measurement system, and • one particular aim is to attenuate any noise in the measurement signal that has not been eliminated by careful design of the measurement system. • Signal filtering, signal amplification, signal attenuation, signal linearization and bias removal are done by signal processing unit. • Signal conditioning has carried out by analogue & digital techniques. • Digital signal conditioning is inherently more accurate than analogue techniques. 2
- 3. Signal Conditioning Functions •Amplification – Increase the level of input signal to better suit. – Improve the sensitivity and resolution of the measurement. • Filtering – Reject useless noise within certain frequency range. – Prevent signal aliasing and distortion. • Attenuation – Contrary to amplification. • Isolation – Solve improper grounding problem of the system. • Multiplexing – Sequentially transmit a number of signals into single digitiser. • Simultaneous Sampling – Issue of measuring more than one signals at the same time. 3
- 4. Analogue signal filtering •Signal filtering consists of processing a signal to remove a certain band of frequencies within it. • The band of frequencies removed can be either at: the low-frequency end the high-frequency end both ends the middle of the spectrum 4
- 5. 5 • Frequency isthe rate of change with respect to time. • Change in a short span of time means high frequency. • Change over a long span of time means low frequency. Frequency and period are the inverse of each other. If a signal does not change at all, its frequency is zero. If a signal changes instantaneously, its frequency is infinite.
- 6. • Filters toperform each of these operations are known respectively as low-pass filters, high- pass filters, band-pass filters and band-stop filters (also known as notch filters). • The range of frequencies passed by a filter is known as the pass-band, the range not passed is known as the stop-band, and the boundary between the two ranges is known as the cut-off frequency. 6
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- 8. 8 Figures 4.1: Outputsfrom ideal filters. Analogue filters exist in two forms, passive and active. Now a days, active filters are used more commonly than passive ones.
- 9. Active analogue filters •The two main difficulties noted on passive filters, were those of obtaining resistance-less inductors and achieving proper matching between signal source and load through the filter sections. • A further problem is that the inductors required by passive filters are bulky and relatively expensive. • Active filters overcome all of these problems and so they are now used more commonly than passive filters. 9
- 10. Signal Processing Functions Thereare many possible functions in the signal-conditioning stage. Some of the common functions are: 1. Signal amplification • Signal amplification is carried out when the typical signal output level of a measurement transducer is considered to be too low. • Amplification is carried out by an operational amplifier. • The operational amplifier is an electronic device that has two input terminals and one output terminal, the two inputs being known as the inverting input and non- inverting input respectively as shown on fig. below. 10
- 11. 11 • The raw(unprocessed) signal Vi is connected to the inverting input through a resistor R1 and the non- inverting input is connected to ground. • A feedback path is provided from the output terminal through a resistor R2 to the inverting input terminal. • Assuming ideal operational amplifier characteristics, the processed signal V0 at the output terminal is then related to the voltage Vi at the input terminal by the expression:
- 12. 2. Signal attenuation •One method of attenuating signals by analogue means is to use a potentiometer connected in a voltage-dividing circuit, as shown in Fig. below. • For the potentiometer slider positioned a distance of x along the resistance element of total length L, the voltage level of the processed signal V0 is related to the voltage level of the raw signal Vi by the expression: 12
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- 14. 3. Differential amplification •used to amplify the small difference that may exist between two voltage signals VA and VB. • These may represent, for example, the pressures either side of an obstruction device put in a pipe to measure the volume flow rate of fluid flowing through it. The output voltage V0 is given by: 14
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- 16. 16 • A differentialamplifier is also very useful for removing common mode noise voltages(Vn). • If the resistance values are chosen carefully such that R4/R2=R3/R1, then the above equation simplifies to: i.e. the noise voltage Vn has been removed.
- 17. 4. Signal linearization •Several types of transducer used in measuring instruments have an output that is a non-linear function of the measured quantity input. • In many cases, this non-linear signal can be converted to a linear one by special operational amplifier Signal linearization. 17
- 18. 5. Bias (zerodrift) removal • Sometimes, either because of the nature of the measurement transducer itself, or as a result of other signal conditioning operations, a bias (zero drift) exists in the output signal. This can be expressed mathematically for a physical quantity x and measurement signal y as: • Where C represents a bias in the output signal that needs to be removed by signal processing. 18
- 19. 6. Signal integration •Connected in the configuration shown in Fig. below, an operational amplifier is able to integrate the input signal Vi such that the output signal V0 is given by: • This circuit is used whenever there is a requirement to integrate the output signal from a transducer. 19
- 20. 7. Voltage follower(pre-amplifier) • The voltage follower, also known as a pre-amplifier, is a unity gain amplifier circuit with a short circuit in the feedback path, as shown in Fig. below, such that: • It has a very high input impedance and its main application is to reduce the load on the measured system. It also has a very low output impedance that is very useful in some impedance-matching applications. 20
- 21. ANALOG AND DIGITAL •Data can be analog or digital. • The term analog data refers to information that is continuous; digital data refers to information that has discrete states. • Analog data take on continuous values. • Digital data take on discrete values. 21
- 22. 22 Data can beanalog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete values. Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values.
- 23. 23 Analog and DigitalSignals Analog Signals • Continuous • Infinite range of values • More exact values, but more difficult to work with Digital Signals • Discrete • Finite range of values • Not as exact as analog, but easier to work with
- 24. 24 Parts of anAnalog Signal Amplitude (peak-to-peak) Amplitude (peak) Period (T) Hz T 1 F Frequency:
- 25. Parts of aDigital Signal 25
- 26. 26 Comparison of analogand digital signals
- 27. Digital signal processing •Digital techniques achieve much greater levels of accuracy in signal processing than equivalent analogue methods. • However, the time taken to process a signal digitally is longer than that required to carry out the same operation by analogue techniques, and the equipment required is more expensive. • Therefore, some care is needed in making the correct choice between digital and analogue methods. 27
- 28. SIGNAL CONDITIONING INSTRUMENTS 1. WHEATSTONEBRIDGE 2. AMPLIFIERS 3. ATTENUATORS 4. FILTERS 5. Integrators 6. Differentiators , etc. 28
- 29. DATAACQUISITION • Data acquisitionis the process of sampling signals that measure real world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. • Data acquisition applications are usually controlled by software programs such as Assembly, BASIC, C, C++, C#, Fortran, Java, LabVIEW, Lisp, Pascal, etc. 29
- 30. DISPLAY AND RECORIDNG INSTRUMENTS •Display Instruments are used for the visual presentation of information's. 1. Analog display devices (cathode-ray tubes) • Oscilloscope tubes •TV CRTs 2. Digital display devices • LED (including OLED) displays • VF (vacuum fluorescent ) displays • LCD (liquid crystal) displays • Nixie tube displays and PDPs (plasma display panels) • Electroluminescent displays (ELDs) 3. Electrical Indicating Instruments, etc. 30
- 31. RECORDERS • A recorderrecords electrical and non-electrical quantities as a function of time. • The record serves the following objectives: 1. It preserves the details of measurement at a particular time 2. It provides at a glance the overall picture of the performance of unit 3. It provides immediate reflection on the action taken by the operator. 31
- 32. • Types ofRecorders: 32
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